The diaryl oxalate is useful for a starting material of a diaryl carbonate by decarbonylation, which has widely been used as a monomer for preparing a polycarbonate by the melting method, and is a compound having attracted attention in recent years. The diaryl oxalate can be prepared by transesterification of a dialkyl oxalate and an aryl alcohol, and the dialkyl oxalate to be used can be prepared from carbon monoxide and an alkyl nitrite.
It has been proposed a sequential process for preparing a diaryl carbonate from carbon monoxide and an alkyl nitrite through a dialkyl oxalate and a diaryl oxalate (see Patent Literature 1).
It has also been proposed a process for preparing a diaryl oxalate which comprises transesterifying a dialkyl oxalate and an aryl alcohol in the presence of a transesterification catalyst, to form an alkylaryl oxalate as an intermediate while removing a by-producing alkyl alcohol by distillation, then, subjecting the alkylaryl oxalate to disproportionation reaction in the presence of a catalyst to form a diaryl oxalate while removing an unreacted dialkyl oxalate by distillation, and recovering the diaryl oxalate by distillating the reaction mixture (see Patent Literature 2).
Further, as a catalyst to be used for preparing a diaryl oxalate by transesterifying a dialkyl oxalate and an aryl alcohol in the presence of a catalyst, it has been proposed to use a titanium compound, a tin compound, a lead compound, a zirconium compound, a molybdenum compound and a ytterbium compound, and among these, a titanium compound has been most preferably used (see Patent Literature 3).
Examples of the titanium compound may be mentioned TiX3, Ti(OAc)3, Ti(OMe)3, Ti(OEt)3, Ti(OBu)3, Ti(OPh)3, TiX4, Ti(OAc)4, Ti(OMe)4, Ti(OEt)4, Ti(OBu)4, Ti(OPh)4 (wherein Ac represents an acetyl group, Me represents a methyl group, Et represents an ethyl group, Bu represents a butyl group, Ph represents a phenyl group, and X represents a halogen atom), and in particular, Ti(OPh)4 [tetra(phenoxy) titanium] is preferred as a catalyst showing excellent transesterification ability.
However, a tetra(aryloxy)titanium such as tetraphenoxy titanium, etc., easily decomposes by reacting with a minute amount of water. Also, the tetra(aryloxy)-titanium is in a solid state at normal temperature, and complicated operations such as solid-liquid separation, etc., are required for purifying the same.
Moreover, it is difficult to use the solid state tetra(aryloxy)titanium for a continuous preparation process of the diaryl oxalate, so that the tetra(aryloxy)titanium is required to be in a liquid state by melting it with heat or dissolving it in a solvent. However, the tetra(aryloxy)titanium is sparingly soluble in the dialkyl oxalate, and has less solubility in an aryl alcohol, which causes increase in a number of processing apparatuses, whereby there is a problem that the preparation steps of the diaryl oxalate are complicated.
The diaryl oxalate obtained by the above-mentioned preparation method, etc., becomes a diaryl carbonate by decarbonylation, and as a final use, it becomes a starting material for a polycarbonate by the melt polycondensation with a bisaryl alcohol A.
The polycarbonate has widely been used for a disc such as CD and DVD, and a plastic lens as an engineering plastic having high transparency. Therefore, to heighten transparency of the resin, it is required not to contain any coloring component for the diaryl carbonate which is a starting material of the polycarbonate, and further for the diaryl oxalate which is a starting material of the diaryl carbonate.
Also, in the above-mentioned preparation method, the reaction and purification by separation are carried out by using a plural steps of a distillation column, but according to the investigation of the preparation method by the present inventors, it was found that corrosion or thinned-down of material was observed at the portion contacting with a reaction mixture, such as the distillation column, storage tank, piping, etc., which are made of an austenitic stainless steel such as SUS304 regulated by JIS G 4304 and generally used for a material of an apparatus such as the distillation column, etc., whereas no compound containing a halogen atom is contained in the reaction mixture, whereby the preparation for a long period of time was difficult.
Furthermore, in the alkyl alcohol removed by the distillation according to the above-mentioned preparation method, impurities with a low boiling point formed at the time of the transesterification are contained, and when they are recycled into the dialkyl oxalate preparation step, unfavorable states that lowering in yield or selectivity of the forming reaction or increase in formed impurities are caused.